Piezo-electric, e.g.quartz crystal, oscillators or resonators are widely used as frequency standards, e.g.for frequency synthesis applications. Whilst the frequency response of a quartz device to temperature changes is relatively small, it is significant in close tolerance applications and must therefore be compensated for by the derivation of an error signal. A preferred approach to this problem is to employ the quartz crystal itself as a temperature sensor. It has been found that some harmonics have a larger frequency response to temperature than others.
One approach to the problem is described for example in U.S. Pat. No. 4,872,765. In this arrangement the correction signal is derived from a signal which is equal to mF.sub.n -n F.sub.m where F.sub.n is the nth harmonic frequency and F.sub.m is the (lower frequency) mth harmonic. This approach suffers from the disadvantage that the relatively large differences between the vibrating areas of the overtone frequencies imposes very severe constraints on the design of a compact crystal which is to provide good performance at the different harmonic frequencies. For example, if the vibrations of a 3.3 MHz fundamental frequency of a 10 um diameter SC-cut crystal are to be confined to the centre of the device in order to avoid unwanted interactions with flexural modes, a surface contour of about 7 dioptre is required. Such a high degree of surface curvature results e.g. in the third overtone mode being excessively sensitive to the effects of small surface irregularities resulting in interaction with flexural modes.
A further approach is to employ harmonics of two different modal classes. This technique is described for example in U.S. Pat. No. 4,468,634 which teaches the use of a tuning fork resonator which vibrates in both a torsioned and a flexural mode. The frequency difference between these two modes is temperature sensitive and thus provides a measure of the crystal temperature. The technique however places severe constraints on the resonator design. Other workers have proposed the use of a B-mode harmonic together with a C-mode harmonic. This can in principle provide accurate temperature measurement. However, it has been found, in crystal cuts where the C-mode is relatively stable, that it is almost impossible to avoid completely coupling between the B-mode and flexural modes. Hence, at certain temperatures, the B-mode exhibits significant distortion in its temperature response. Further, the frequency of the B-mode oscillator has been found to be sensitive to stress associated with the rate of temperature change thus introducing further inaccuracy.
An object of the present invention is to minimise or to overcome these disadvantages.
A further object of the invention is to provide a piezoelectric resonator temperature sensor having a high accuracy and exhibiting stable operation.